1. Field of the Invention
The present invention relates in general to a method of making ZSM-5 zeolite. More specifically, the present invention relates to using plant waste material such as rice hull ash as a source for SiO2 in making ZSM-5, thus producing a ZSM-5 that is lower cost than prior art methods.
2. Description of the Prior Art
Zeolites are one of the most important classes of aluminosilicates. A zeolite may be defined as an aluminosilicate with a framework structure enclosing cavities occupied by large ions and water molecules. The framework consists of an open (Alxe2x80x94SiO2)∞ arrangement of corner-sharing tetrahedra plus enough cations to give electroneutrality and several water molecules to occupy the cavities.
The structure of zeolites is what gives them unique qualities. Synthetic zeolites are used, for example, as adsorbents in separation processes, as replacements for phosphates in detergents, and as components in catalysts in the petrochemical industry for converting hydrocarbons into other useful products. The porous structure of the zeolite allows different sized molecules to enter the pores, and the hydrophobic or polar interactions with those molecules act as a catalyst in that molecule""s conversion to another useful target compound, either through rearrangement, addition, or extraction reactions. A larger pore might have a stronger affinity for larger chain hydrocarbons or highly branched hydrocarbons, for example, while small pores may have a higher affinity for smaller molecules such as water or ions. Thus, the size of the pore, or cavity size in combination with acidity can alter the catalytic properties of the zeolite.
The various stoichiometries of SiO2, Al2O3, and other oxides lead to various zeolites. One such zeolite that is of great interest is called zeolite Socony Mobil-5 (SM-5), or simply ZSM-5. The preparation of ZSM-5 and its structure is disclosed in U.S. Pat. No. 3,702,886, the entire disclosure of which is incorporated herein by reference. The earliest ZSM5s were prepared using an organic template in the synthesis mixture which directs the formation of the ZSM-5 pore structure. The final structure of a ZSM-5 zeolite has a lattice configuration which encompasses three basic functional groups: Al2O3 and SiO2, and Na2O. Thus, ZSM-5s are often described in terms of these functional groups and their relative ratios within the zeolite lattice. The ratios of these groups, especially the SiO2/Al2O3 molar ratio, is an important indicator of the useful properties the zeolite will possess. For instance, for catalytic cracking operations, zeolites are most useful with a the range of 15-150 SiO2/Al2O3 molar ratio. Acid resistance and thermal stability also increase as this ratio increases. On the other hand, for adsorption and cation exchange uses, a decrease in the SiO2/Al2O3 molar ratio increases the efficiency due to an increase in cations in the zeolite lattice.
Along with changing the SiO2/Al2O3 molar ratio, control of the pore size and the general lattice structure of zeolites is often accomplished by use of templates such as tetrapropylammonium (TPA) salts. ZSM-5 zeolites prepared using an organic template are common, and generally have SiO2/Al2O3 molar ratios of at least 60, and frequently greater. ZSM-5s can also be made using an inorganic base such as NaOH. Using NaOH, SiO2/Al2O3 molar ratios of from 20 to 30 have been achieved. Further, the inorganic cations present influence the zeolite lattice framework and useful properties. While cations are necessary, if for no other reason, for charge balance, the identity of the cation can be chosen to tailor the zeolite. For ZSM-5 zeolites, Ba, Na, K, Li, Cs, NH3+and mixtures thereof have been used in its synthesis.
There are several disadvantages to using organic templates. Since the synthesis of the ZSM-5 zeolite must be carried out under pressure while heating, these organic templating agents are a safety and/or environmental hazard once released into the atmosphere or water system. Further, these compounds add to the cost of producing ZSM-5. For instance, TPA alone often makes up as much as 50% of the production costs of ZSM-5 in prior art processes. Removing the templating agent, which is typically accomplished by combustion of the ZSM-5, can result in the degradation of the lattice structure of the zeolite. Thus, a process for producing ZSM-5 that does not require a template is highly desirable. One process of making ZSM-5 that does not use a template is disclosed by Thome et al. in U.S. Pat. No. 5,385,714, the entire disclosure of which is herein incorporated by reference.
Although the method of making ZSM-5 disclosed by Thome et al. excludes the use of templates, the cost of other starting materials is also a factor in the usefulness of zeolite catalysts. Thus, a further disadvantage with the prior art methods of manufacturing ZSM-5 and other zeolites is the cost of the primary starting materials. Reducing the cost of starting materials such as SiO2 would further enhance the usefulness and commercial viability of ZSM-5, making its use possible in a greater range of industry and commerce. The present invention is directed to a method of using plant waste material such as rice hull ash or other siliceous wastes containing amorphous SiO2 (plant and geothermal sludge, etc.) as a source for SiO2 in making ZSM-5, and compositions thereof, both of which exclude the use of an organic templating agent.
One object of the present invention is to provide a method for the manufacture of a ZSM-5 from siliceous ash derived from waste plant material including siliceous plant residue from bio-gas operations, geothermal sludge, or any other siliceous waste containing amorphous SiO2 or alumino-silicates, thus of relatively low cost and of ecological value.
Another object of the present invention is to provide a method of manufacturing a ZSM-5 with a carbon content between about 0% and 10% or more.
Another object of the present invention is to provide a method of manufacturing ZSM-5 that is not dependent upon the use of an organic templating agent.
Yet another object of the present invention is to provide a method of manufacturing ZSM-5 that allows an economically viable usage of the zeolite in wide ranging applications such as the controlled release of chemical compounds, deodorizers, toxic-waste cleanup, permeable barriers, and situations where activated carbon cannot be used, as well as traditional use in catalytic cracking of hydrocarbons and other catalytic processes.
These and other objects of the present invention are achieved by providing an improved method of manufacturing a ZSM-5 zeolite. The method comprises providing siliceous ash having a carbon content between about 0% and 10% by weight of total rice hull ash, wherein the SiO2 content is greater than 90% by weight of total siliceous ash, and wherein the SiO2 and carbon are substantially amorphous and/or graphitic. The synthesis of the ZSM-5 of the invention involves several steps, the exact order of which are not important, but are described as steps for convenience only. The first step is suspending the siliceous ash in water. Next, a source of alumina is added to the suspension of siliceous ash. The source of alumina and siliceous ash are added such that the molar ratio of SiO2 to Al2O3 in the ZSM-5 is in the range from approximately 15 to 150, thus forming a second suspension. Sodium Hydroxide is then added to the second suspension, wherein the quantity of NaOH added is such that the molar ratio of Na2O to Al2O3 in the ZSM-5 is in the range of about 2-10, and preferably about 7.4, thus forming a third suspension. In order to precipitate the ZSM-5, the third suspension is seeded with highly pure SiO2, silicalite, or ZSM-5 and heated in a closed system, thus yielding the ZSM-5.
The ZSM-5 of the invention is made from a siliceous ash that is characterized by having SiO2 and carbon either amorphous and/or graphitic. Amorphous is desirable. The siliceous ash source can be any source of amorphous SiO2 such as rice hulls and other plant waste materials as well as geothermal siliceous waste, siliceous sludge, rice hull waste from bio-gas production, etc, thus termed siliceous waste material. Further, the source of alumina and siliceous ash are added such that the molar ratio of SiO2 to Al2O3 in the ZSM-5 is in the range of approximately 15 to 150. Also, the NaOH is added such that the ratio in the ZSM-5 of Na2O to Al2O3 is between about 2 and 10. Excess carbon can be removed by heating the ZSM-5 at 450xc2x0 C. to 550xc2x0 C. for 6 to 12 hours. However, the presence of amorphous carbon may be desirable in enhancing the hydrophobicity of the zeolite.
Additional objects, features and advantages will be apparent in the written description which follows.